Mine does exactly the same thing, I complained to the dealer and they kept doing software updates and told me that sense no code was showing there was nothing else they could do about it. That was at approximately 60,000 miles it now has 120,000 and still does the same thing. I did call call BMW corporate... they were morons.

It's amazing the reliance BMW technicians have on the computer to diagnose a problem. There are plenty of things that can go wrong with these vehicles which are not monitored by the computers.

Take Graham132white's recommendation of looking into the rear chassis bushings. To my knowledge rear chassis bushings are not monitored by the computer. And if he's right about these bushings being the problem BMW technicians will never know as the computer didn't tell them to examine them.